UNITED NATIONS, Aug. 25 (Xinhua) -- UN Secretary-General Ban Ki-moon on Monday called on the warring parties in South Sudan to "immediately implement" an agreement on the cessation of hostilities and "uphold their commitment to establishing a Transitional Government of National Unity."

The secretary-general, in a statement issued here by his spokesman, said that he noticed the South Sudan government and the Sudan People's Liberation Army in Opposition signed earlier Monday the Implementation Matrix of the Jan. 23 Cessation of Hostilities Agreement.

The signing took place in Ethiopia's capital of Addis Ababa during the summit of the Inter-Governmental Authority on Development (IGAD) in Eastern Africa on South Sudan from Sunday to Monday.

"The secretary-general commends the IGAD heads of state and their special envoys for their tireless efforts to find a peaceful resolution to the crisis in South Sudan and reiterates the full support of the United Nations to this process," the statement said.

IGAD leaders convened their summit as the regional bloc continued its efforts to mediate peace talks aimed at resolving the South Sudan crisis.

The summit, chaired by Ethiopian Prime Minister Hailemariam Desalegn, was also attended by presidents of Uganda, Djibouti, South Sudan and Kenya as well as Sudan's first vice president and Somalia's prime minister.

In mid-December 2013, a full-fledged conflict sprang from a political impasse between President Salva Kiir and former Vice President Riek Machar, leaving an estimated 1.5 million people in South Sudan uprooted and on the brink of a humanitarian catastrophe.

Nearly 100,000 civilians have also been forced to flee to UN peacekeeping bases around the country.

Earlier this month, a delegation of the UN Security Council visited South Sudan and met leaders of the government and the opposition, aiming to highlight the importance of organizing a governing body in the run-up to the election scheduled for next year. The deadline for organizing such a body passed on Aug. 10.

Ban and the Security Council have expressed warnings and concerns about the deteriorating security situation and looming humanitarian catastrophe in South Sudan.

The secretary-general and the 15-nation UN body maintain that a military solution in South Sudan is not acceptable, while urging both Kiir and Machar to fully engage in the ongoing peace talks and uphold their commitment to establishing a transitional government.